The present invention relates to orthopedic devices, and more particularly to devices know as external fixators or skeletal fixators. These devices are used, as an alternative to plaster casts or a combination of surgically implanted screws, pins and plates, to reposition two tissue segments, e.g. bone elements, relative to each other. More specifically, the invention relates to an adjustable strut for use as a component of such a fixator. Additional background information relating to such devices can be found in U.S. Pat. No. 5,702,389, having the same inventors.
Skeletal injuries or conditions are sometimes treated with an external fixator that is attached to the boney skeleton with threaded and/or smooth pins and/or threaded and/or smooth or beaded wires. External fixators may be utilized to treat acute fractures of the skeleton, soft tissue injuries, delayed union of the skeleton when bones are slow to heal, nonunion of the skeleton when bones have not healed, malunion whereby broken or fractured bones have healed in a malposition, congenital deformities whereby bones develop a malposition, and corrective bone lengthening, widening, or twisting.
External fixators vary considerably in design and capabilities, and may include multiple or single bars or rods, and a plurality of clamps for adjustably securing the bars to pins or wires which are, in turn, joined to the boney skeleton. The pins or wires may extend completely through the boney skeleton and protrude from each side of the limb, or may extend through the boney skeleton and protrude from only one side of the limb. Pins which extend completely through the boney skeleton and protrude from both sides of a limb are commonly referred to as "transfixation pins." Pins which extend through the boney skeleton and protrude from only one side of the limb are commonly referred to as "half pins."
External fixators may be circumferential, encircling a patient's body member (e.g., a patient's leg), or may be unilateral, extending along one side of a body member. More than one unilateral external fixator can be applied to the same portion of the patient's body member. Materials from which fixators are constructed vary, including metals, metal alloys, plastics, composites, and ceramics, as is known to those skilled in the art. External fixators also vary considerably in their ability to accommodate different spatial relations between the relevant tissue segments.
One of the more commonly used types of external fixators was described by G. A. Ilizarov during the early 1950's. The Ilizarov system includes two or more rings or "halos" that encircle a body member, connecting rods extending between the two rings, transfixion pins that extend through the patient's boney structure, and connectors for attaching the transfixion pins to the rings. Use of the Ilizarov system to address problems of angulation, translation and rotation is disclosed in "Basic Ilizarov Techniques," Techniques in Orthopaedics.RTM., Vol. 5, No. 4, December 1990, pages 55-59.
The Ilizarov system allows a physician to reorient one tissue fragment with respect to another along six axes in an acute motion. However, this system is disadvantageous in that it utilizes hinges and translation mechanisms which must be specifically constructed for a given case. Considerable planning, fabrication and preparation is required to use the device because it utilizes different mechanisms depending on the translational or rotational corrections a bone is required to undergo. Moreover, it requires that the physician loosen one or more clamps, apply corrective motion manually, and then retighten the clamps to hold the fragments stably each time a new or adjusted bone position is required. Aside from the disadvantages of inconvenience and imprecision, this system has the further undesirable effect of necessarily subjecting the skeletal tissues to abrupt motion, sometimes along an indirect path.
The mathematics describing the positional relations of tissue segments are best illustrated by the "Chasles axis theorem." Chasles recognized that the complex repositioning of an object with respect to six axes (three rotational and three translational) could be duplicated by the rotation of a threaded nut along a threaded shaft. This theoretical shaft is oblique to an arbitrary reference axis. The offset from the center of the shaft (i.e., the thickness of the imaginary shaft) will satisfy two translational components and the pitch of the thread satisfies the third translation. Rotation around the oblique shaft is the equivalent to a combination of three orthogonal rotations. This modeling of six repositioning elements serves as a useful model for understanding orthotic deformity correction where a skeletal element must be translated and rotated to restore its correct position. Specifically, by taking advantage of this oblique Chasles axis, all three mal-rotations and all three mal-transitions of a deformity can be corrected simultaneously.
The essential elements of a more recently developed device (referred to hereinafter as the earlier Taylor device) were described in a publication entitled "The Taylor Spatial Frame Fixator" by Dr. J. Charles Taylor M.D. (see also U.S. Pat. No. 5,702,389). This fixator, which utilizes the Chasles axis theorem, consists of two base elements, usually rings or partial rings, connected by at least six telescopic struts. At least three connection points are selected on each ring, and the struts are connected to connection points in series (thereby defining the effective plane that is manipulated by the struts).
The earlier Taylor device manipulates the adjacent bone tissues through the predicted effect an adjustment of each of the six struts will have on the base members. Through translation and rotation analysis of one of the tissue segments relative to the other, a desired position of the base members can be calculated. Geometric principles are applied to the initial and final position of the strut ends to calculate the final strut length.
The earlier Taylor device can best be used in one of two modes. In the first mode, known as the chronic method, the base members mimic the deformity in the bone tissues before being brought back the a neutral position. In other words, after an initial aligned and parallel position for the base members is determined, one of the base members is translated in the same direction, and rotated about the same point, as the deformed bone member has been translated and rotated from the target position. The fixator is then attached to the bone segments and brought back to its initial aligned position, thereby reorienting the bone members as desired. The second mode of operation, the acute method, works in a manner essentially opposite to the chronic method. The device is attached to the tissue segments in a neutral position. Calculations are then made to determine the translational and rotational requirements for the base members to mirror the bone tissue deformity. Thus when the base members are brought to this mirroring position, the tissue segments will be in alignment, with the base members in an unaligned position.
The connecting interface between each strut and the end plates in the earlier Taylor device consists of a captured bifurcated ball joint. This connection provides four degrees of freedom (i.e., three orthogonal degrees of strut rotation relative the base member and one degree of rotation about an axis perpendicular to the face of each hemisphere). Adjustment is accomplished with a turnbuckle-style shaft which is threaded about a rod at either end, with the direction of the threads at one end being reversed relative the other end. Thus rotation of the turnbuckle causes the rods, and thus the base members, to expand or contract.
Despite the important advances embodied in the earlier Taylor device, its performance could be more advantageous in some situations. The captured ball system is not ideal for application to an orthopedic fixator because it tends to bind as the fixator is forced to deform beyond certain limits. In other situations, the rotation of the captured ball relative the strut could cause small changes in the length of the strut. Moreover, the turnbuckle style strut used in the earlier Taylor device is free to rotate about the adjustment rods, thereby creating a risk that the relative position of the base members can drift from the intended position.
The earlier Taylor device is also relatively difficult to use in situations in which the struts must be removed. In some cases the translation of the tissues will cover a considerable distance or angle, requiring commensurate extension or retraction of the struts. Thus, it is sometimes necessary to replace one or more struts with struts of a different size during a clinical procedure. Moreover, even after the connector has been disassembled, the permanent indications of position on the earlier Taylor device make it more difficult to remove and replace struts, since each strut is designed for a specific position on the frame.
When using the earlier Taylor devices, it is sometimes difficult to determine the effective length of the strut. The device does provide slots through the side of the turnbuckle through which the position of the internal rod can be seen. However, since the shaft on the turnbuckle has to be rotated to see in the slot, the act of checking the position can result in an undesired or unintended change in position.
In light of the foregoing, it will be appreciated that there is still a need for an improved telescopically adjustable strut for use with an external orthopedic fixator. Ideally, the strut should not bind against the base members when the components form relatively sharp angles, it should visibly indicate the effective length of the struts without requiring that the length be altered to observe the indication, it should be capable of rotating axially without altering its effective length, it should indicate the strut's position on the fixator yet remain interchangeable with other fixators at other positions on the device, and it should be axially adjustable without requiring clamps yet should not permit drift from the desired effective length.